CELLS AND THEIR ASSOCIATION 41 



where the cells form but a very small fraction of the 

 total m;i 



It was suggested a moment ago that blood is a tissue. 

 This may not be a familiar idea nor one that is universally 

 acceptable. But it will be noted that blood conforms 

 with the terms of our definition. It contains cells of 

 standard kinds with intervening material which in this 

 instance is a liquid. There is more or less intercellular 

 fluid in almost any variety of tissue. 



Four conspicuous orders of tissues may be set apart. 

 They are (1) the epithelial, (2) the connective, (3) the 

 contractile, and (4) the nervous. Those of the first class 

 (epithelial tissues) are the surface tissues, the coverings 



FIG. 3. To emphasize the difference between an epithelial tissue 

 (at the left) in which the cells are closely packed and a form of connec- 

 tive tissue- (cartilage) consisting mainly of an intercellular deposit. 



and linings of the organs. An epithelium may consist 

 of a single layer of cells or of more than one. The thin- 

 nest developments of this character are exemplified in 

 the partitions between the air and the blood in the 

 lungs. In calloused portions of the skin the number of 

 layers of cells is large and those on the outer surface are 

 lifeless, flattened, and dry. Epithelial cells, as a rule, 

 keep growing and subdividing throughout life to make 

 good the loss by degeneration and detachment that is 

 constantly going on. 



Connective tissues have the general function which the 

 name indicates. Bone is an example and what the 

 skeleton does for the body as a whole is done for each 



